This invention relates to apparatus for polishing semiconductor or similar type materials, and more specifically to such apparatus which facilitates equalization of the downward pressure over the polished wafer surface and the polishing head of the apparatus.
Polishing an article to produce a surface which is highly reflective and damage free has application in many fields. A particularly good finish is required when polishing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process (hereinafter xe2x80x9clithographyxe2x80x9d). Flatness of the wafer surface on which circuits are to be printed is critical to maintain resolution of the lines, which can be as thin as 0.13 microns (5.1 microinches) or less. The need for a flat wafer surface, and in particular local flatness in discrete areas on the surface, is heightened when stepper lithographic processing is employed.
Flatness is quantified in terms of a global flatness variation parameter (for example, total thickness variation (xe2x80x9cTTVxe2x80x9d)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (xe2x80x9cSTIRxe2x80x9d) or Site Focal Plane Deviation (xe2x80x9cSFPDxe2x80x9d)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane, referred to as the xe2x80x9cfocalxe2x80x9d plane. SFQR is a specific type of STIR measurement, as measured from the front side best fit reference plane. A more detailed discussion of the characterization of wafer flatness can be found in F. Shimura, Semiconductor Silicon Crystal Technology 191-195 (Academic Press 1989). Presently, flatness parameters of the polish surfaces of single side polished wafers are typically acceptable within a central portion of most wafers, but the flatness parameters become unacceptable near the edges of the wafers, as described below.
Polishing machines typically include an annular polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on pressure plates above the polishing pad and lowered into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad for greater polishing interaction between the polishing pad and the wafer.
In order to achieve the degree of polishing needed, a substantial normal force presses the wafer into engagement with the pad. The coefficient of friction between the pad and wafer creates a significant lateral force on the wafer. This lateral force can give rise to certain distortions in the polish, such as by creating a vertical component of the frictional force at the leading edge of a wafer. The vertical component of the frictional force is created because the wafer is mounted to pivot about a gimbal point under influences of the lateral friction forces. A change in the net vertical force applied to the wafer locally changes the polishing pressure and the polishing rate of the wafer, giving rise to distortions in the polish. Often the uneven forces cause the wafer""s peripheral edge margin to be slightly thinner than the majority of the wafer, rendering the edge margin of the wafer unusable for lithographic processing. This condition is a sub-species of the more general problems associated with wafer flatness, and will be referred to hereinafter as edge roll-off.
Improvements in wafer polishers have helped reduce edge roll-off. Recent configurations have incorporated conic bearing assemblies between the wafer and the mechanism applying the polishing force, while permitting free rotation of the wafer. Conic bearing assemblies are an improvement over traditional ball and socket configurations because the gimbal point of the mechanism is at a point below the bearing, nearer the interface between the wafer and the polishing pad. Wafers polished with a gimbal point near the work surface exhibit superior flatness characteristics, particularly near the outer edge of the wafer where conventional polishing processes exhibit characteristic xe2x80x9croll-offxe2x80x9d and near the center of the wafer where slurry starvation may occur.
Another improvement directed toward more uniform wafer polishing is the use of a membrane to apply pressure to the rear surface of the wafer. Because membranes rely on air pressure to exert force upon the wafer, the pressure is thought to be more uniform over the wafer surface throughout the polishing process. Membranes, however, suffer from drawbacks. First, membranes must stretch during inflation to apply pressure over the wafer. Because the entire membrane must stretch as it attempts to engage the wafer, a portion of the pressure is used to stretch the wafer, instead of applying pressure to the wafer. Moreover, as the central portion of the membrane stretches toward the wafer, the lateral edges of the membrane are held tightly and cannot stretch enough to fully engage the wafer. By stretching the central portion only, while inhibiting the lateral edges of the membrane from engaging the wafer, the membrane provides inadequate support at the wafer""s edge. Thus, the pressure applied at the edge of the wafer is due to the stiffness of the wafer itself, rather than from engagement with the membrane, causing the wafer edge to be underpolished. Secondly, if the rotational speed of the wafer and polishing pad become unsynchronized, torque is created on the wafer. Such torque can wrinkle the membrane, leading to uneven polishing or catastrophic failure, as the wafer may slip out of the polishing head during polishing. Thus, a configuration is needed incorporating further features for facilitating wafer flatness due to more uniform polishing, while overcoming the drawbacks mentioned above.
Among the several objects and features of the present invention may be noted the provision of a semiconductor wafer polishing apparatus, method and polishing head which apply uniform polishing pressure over the surface of the wafer; the provision of such an apparatus, method and head which facilitate better polishing pressure near the lateral edge of the wafer; and the provision of such an apparatus, method and head which provide efficient pick-up and release of the wafer from the polishing head.
Generally, a wafer polishing apparatus of the present invention for polishing a front surface of a wafer comprises a base for supporting elements of the polishing apparatus. A turntable mounts on the base for rotation about an axis on the base and is adapted to support a polishing pad for conjoint rotation with the turntable. The polishing pad has a work surface engageable with the front surface of the wafer for use in polishing the front surface of the wafer. A turntable drive mechanism operatively connects to the turntable for selectively driving rotation of the turntable about the axis of rotation. A polishing head mounts for holding the wafer in generally opposed relation with the turntable and for rotation about an axis generally parallel to the axis of rotation of the turntable. The polishing head includes a back plate having at least a central region in opposed relation with a rear surface of the wafer when the wafer is received by the polishing head. An annular sealing ring of flexible material has a thickness and is disposed around the central region of the back plate. The sealing ring has a central opening extending through the complete thickness of the sealing ring and is disposed for engaging a peripheral edge margin of the wafer, such that the rear surface of the wafer, the sealing ring and the back plate define a substantially fluid-tight cavity for controlling fluid pressure in the cavity.
In yet another embodiment of the present invention, a method of polishing a semiconductor wafer comprises placing a rear surface of the semiconductor wafer in engagement with a seal of the polishing head of a wafer polishing apparatus to form a fluid pressure cavity defined by the rear surface of the wafer, the seal and the polishing head. The wafer is mounted on the polishing head by evacuating the fluid pressure cavity to draw the wafer to the polishing head and hold the wafer. The method further comprises engaging a front surface of the wafer on the polishing head with a polishing pad on a turntable and urging the front surface of the wafer against the polishing pad by selectively applying air pressure within the cavity for pressing the wafer surface uniformly against the polishing pad. Air within the cavity directly engages a majority of the rear surface of the wafer. The wafer is disengaged from the turntable and removed from the polishing head.
The present invention is also directed to a polishing head generally set forth as above.
The present invention is also directed to a method of processing a semiconductor wafer. An oxide layer is formed on a rear surface of the semiconductor wafer. The semiconductor wafer is then free-mounted on a polishing head of a wafer polishing apparatus. A front surface of the wafer on the polishing head engages a polishing pad on a turntable. Relative motion between the wafer and the polishing pad is obtained, and the front surface of the wafer is urged against the work surface. The wafer is removed from the polishing head.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.